Acetone smoothing has two uses. One is just to get a smooth surface (duh) which one may want for the looks (when I make a dark bronze painted and bluish patina'ed figurine for example).

The other use is technical as the smoothing makes a totally water- and gas- tight surface (which I need in my projects). Also, the smoothing process creates a totally homogenous surface layer. When applying pull-stresses, there are no inter-layer crevices (which will be there at any layer height, even 0.1mm) at the surface which can be initiating points for cracks to develop as the forces are the largest there. The smoothed ABS part thus has an enormously increased resistance to pull-stress cracking. Parts that endure torsion (one side press-stress, the other side pull-stress) are thus stronger.